The removal of organic coatings or residues from a substrate, and particularly the removal of such organic coatings or residues as greases, oils, mold release coatings, polyester coatings, epoxy coatings, paints and other types of coatings, is extremely important in many industries. In particular, organic coatings or residues need to be removed from substrates either for the re-working of a part which has a flawed coating or for reclaiming parts in which the substrate is intact but which requires residues to be removed and/or new coatings to be applied.
It is known that caustic compounds used at elevated temperatures in a fused, essentially anhydrous condition are very effective in removing many types of organic coatings and residues from many types of metal substrates. One such method is taught by U.S. Pat. No. 3,790,489 to Shoemaker et al. However, certain metals, such as zinc, aluminum and magnesium or metallic coatings comprised of these metals or alloys thereof, as well as other metals, alloys, and even non-metallic substrates, are subject to chemical attack or destruction under such conditions as taught in the '489 patent. Thus, such caustic compositions are typically avoided in prior art teachings when removing coatings from metals, such as aluminum, magnesium and zinc.
One prior art approach which avoids the use of a caustic solution is taught by U.S. Pat. No. 5,894,854 to Miles, wherein nonferrous substrate stripping is achieved by using a mixture of triethanolamine and an alkylphenol ethoxylate surfactant at elevated temperatures of about 350° F. While this method is effective for stripping some types of coatings from nonferrous substrates without causing substantial harm to the substrate, this method is unacceptably slow or even ineffective for some applications. Certain coatings are sufficiently chemically resistant that they cannot be efficiently stripped, even with additional post-treatment steps, such as rinsing and pressure spraying.
Another approach is taught by the U.S. patent application of John F. Pilznienski et al. for METHOD AND COMPOSITION FOR REMOVING ORGANIC COATINGS FROM A SUBSTRATE, Ser. No. 10/058,675, filed Jan. 29, 2002, published on Jul. 31, 2003 under Publication No. US-2003-0144164-A1. A composition taught therein includes an ethanolamine organic carrier, non-ionic alkylphenol ethoxylate surfactants and mixtures thereof and potassium hydroxide. The composition is further characterized by being essentially free of water or having a water content low enough so the hydroxide is not ionized to an extent that it will attack zinc, aluminum or magnesium substrates or coatings of such metals.
However, while the Pilznienski et al method is effective in stripping many types of coatings, some coatings are extremely resistant and are not effectively stripped, such as “clearcoat” painted surfaces commonly found on automotive parts, wherein a base or color layer is topped off with a “clearcoat” or “topcoat” layer and cured. Once cured, the clearcoat painted surface provides very good chemical and environmental resistance to corrosion, damage, fading, and permeation of the painted surface. Unfortunately, this superior chemical and environmental degradation resistance also provides significant resistance to stripping. Specifically, clearcoats are formulated to withstand gasoline spills and other chemical exposures that may occur at typical outside environmental temperatures. Accordingly, low temperature applications of stripping mixtures are not likely to break down the clearcoat paint surface and, thus, elevated temperatures must be used to break down the outer shell. Although clearcoated surfaces can be stripped, this requires longer exposure times, and prior art stripping mixtures, such as those taught by Pilznienski et al, must be made more aggressive, either by raising bath temperatures or increasing the potassium hydroxide (KOH) above the preferred values taught therein, or both. Increasing the KOH content of the Pilznienski et al stripping solution adds moisture—even “solid” KOH has about 10% water—and also potentially increases its hygroscopicity; and it has been found that the presence of water within the solution can result in attack of more sensitive or reactive substrates, for example, damage to galvanized metal substrates. And although, in some applications, the mixture may be manipulated to minimize underlying substrate damage, stripping times must then be correspondingly increased greatly for some resistant coatings, resulting in unacceptably long strip times.
Moreover, foaming problems may also arise in the application of a stripping mixture where high wetting agent (surfactant) levels are required or indicated. Foaming problems arise not during application of the prior art stripping mixture itself to an item to be stripped, but during a subsequent water rinse step in the stripping process. Loosened coatings and coating remnants broken down by the stripping mixture are typically removed through the application of water, along with residual stripping mixture, typically through spray washing or a water-rinse bath immersion. Accordingly, steps must be taken to counteract the foaming, resulting in undesirable process problems and inefficiencies.
U.S. Pat. No. 3,954,648 to Belcak et al provides another approach to stripping coatings from various metallic surfaces. Belcak et al teach a solution with three key ingredients—a high-boiling oxygenated solvent (a significant difference and departure from the Pilznienski et al stripping solution teachings), a high boiling liquid amine, and a smaller amount of an alkali metal hydroxide. Belcak et al also suggest the use of a surface active wetting agent if needed. Belcak et al provide a large amount of test data, including various chemical blends, coating types tested, and the test results. However, experiments with solutions according to these teachings as applied in the removal of clearcoat paint coatings also result in unsatisfactory results. In the first place, foaming issues arise unless recommended surfactant amounts are significantly lowered, which inhibits the stripping capacity of the solution. And, significantly, stripping times were unacceptably long for efficient application, with some times exceeding one hour. If more robust solutions are selected from the Belcak et al teachings to address these problems, then substrate damage appears. Numerous experiments with different amine levels invariably return unsatisfactory results in clearcoat stripping, with either unacceptable times or galvanized substrate damage issues.
What is needed is a composition and method which will effectively remove resistant organic surface coatings, such as clearcoat paint, from substrates in a timely fashion without attacking and causing unacceptable damage to the underlying substrates.